Zagros orogeny: a subduction-dominated process Philippe Agard, Jafar Omrani, Laurent Jolivet, Hubert Whitechurch, Bruno Vrielynck, Wim Spakman, Patrick Monié, Bertrand Meyer, R. Wortel To cite this version: Philippe Agard, Jafar Omrani, Laurent Jolivet, Hubert Whitechurch, Bruno Vrielynck, et al.. Zagros orogeny: a subduction-dominated process. Geological Magazine, Cambridge University Press (CUP), 2011, 148 (5-6), pp.692-725. 10.1017/S001675681100046X. insu-00616582 HAL Id: insu-00616582 https://hal-insu.archives-ouvertes.fr/insu-00616582 Submitted on 4 Feb 2013 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Geol. Mag. 148 (5–6), 2011, pp. 692–725. c Cambridge University Press 2011 692 doi:10.1017/S001675681100046X Zagros orogeny: a subduction-dominated process P. AGARD∗†, J. OMRANI‡, L. JOLIVET§, H. WHITECHURCH¶, B. VRIELYNCK∗, W. SPAKMAN,P.MONIÉ#,B.MEYER∗ &R. WORTEL ∗ISTEP, UMR 7193, Université Pierre et Marie Curie – Université Paris 6, 4 pl. Jussieu, Paris 75005, France ‡Geological Survey of Iran, Tehran, Iran §ISTO, Université d’Orléans, France ¶EOST, Strasbourg, France Geosciences, Utrecht University, The Netherlands #Géosciences Montpellier, UMR 5573, Université Montpellier 2, France (Received 17 January 2011; accepted 3 May 2011; first published online 5 July 2011) Abstract – This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ∼ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ∼ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (∼ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ∼ 12– 8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (∼ 10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes. Keywords: Zagros, geodynamics, orogeny, subduction, arc magmatism, obduction, slab break-off. 1. Introduction As for other segments of the Alpine–Himalayan belts, the Zagros collision zone formed as a result The Zagros orogen stands at the tectonic crossroads of the disappearance of the Neo-Tethys Ocean (e.g. of the Alpine–Himalayan belts. Its formation results Takin 1972; Ricou, Braud & Brunn, 1977; Alavi 1980, from the long-standing convergence between Eurasia 1994; Berberian & King, 1981; Dercourt et al. 1986; and Gondwanian-derived fragments, as underlined by Dercourt, Ricou & Vrielynck, 1993; Sengör et al. ophiolite belts or present-day GPS vectors (Fig. 1a). 1988; Stampfli & Borel, 2002; Agard et al. 2005; The Zagros orogen hosts considerable oil resources Hafkenscheid, Wortel & Spakman, 2006) between and spectacular, world famous whaleback folds. Yet, Arabia and Eurasia (Fig. 1b). The timing of collision, despite a wealth of recent data on GPS displacements, however, has been highly controversial, ranging from active deformation and kinematics (e.g. Vernant et al. Late Cretaceous (Berberian & King, 1981) to Miocene 2004; Walker, 2006; Le Dortz et al. 2009; Allen et al. (Berberian & Berberian, 1981) or uppermost Pliocene 2011; Reilinger & McClusky, 2010) or on the tectono- (Stöcklin, 1968), to only mention cornerstone papers. sedimentary record (e.g. Sherkati, Letouzey & Frizon Although there is a growing body of evidence in support de Lamotte, 2006; Mouthereau et al. 2007; Homke of Late Eocene to Oligocene initial collision (e.g. et al. 2009, 2010), the Zagros orogen remains much Jolivet & Faccenna, 2000; Agard et al. 2005; Vincent less documented than its adjacent counterparts (the et al. 2005; Ballato et al. 2010 and references therein), Himalayas, Turkey or the Alps). very contrasting interpretations are still published (e.g. Ghalamghash et al. 2009; Mazhari et al. 2009b). The position of the suture zone itself, regarded by †Author for correspondence: [email protected] most authors to lie along the Main Zagros Thrust Zagros orogeny: a subduction-dominated process 693 Figure 1. (Colour online) (a) The Zagros orogen: a central segment (at the tectonic crossroads) of the gigantic Alpine convergence zone from the Himalayas to the Western Mediterranean region. Present-day convergence underlined by GPS vectors (after Vernant et al. 2004). Finite convergence is underlined by ophiolites (in black; after Khan et al. 2006) sandwiched between the major tectonic domains and high-pressure, low-temperature rocks (red stars; mostly blueschists) returned from the Neo-Tethyan suture zone. Topography as background greyscale. Frame shows location of (b) and (c). (b) Simplified palaeogeographic reconstruction for the Neo-Tethys during Late Cretaceous time (c. 95 Ma; after Vrielynck & Bouysse, 2003; Barrier & Vrielynck, 2008). The location (and regional extent) of blueschist exhumation in the NSZ (Neo-Tethyan subduction zone) is compared with that of obduction to the south of the Neo-Tethys. (c) Simplified geological map of Iran showing the main tectonic subdivisions and locations discussed in the text. Also shown are the two transects (Kermanshah, Anar) whose lithospheric-scale reconstructions are shown in Figure 10 (corresponding tomographic sections are shown in Fig. 9). Abbreviations: K – Kermanshah; MZT – Main Zagros Thrust; N – Neyriz; NB – Nain–Baft; SB – Sabzevar; SO – Sistan ocean; SSZ – Sanandaj–Sirjan Zone; UDMA – Urumieh–Dokhtar Magmatic Arc; ZFTB – Zagros fold-and-thrust belt. (d) Present-day deformation of Iran, as seen through seismicity (yellow dots) and the location of major active faults (for deformation partitioning, see also Fig. 13a). GPS vectors shown in red. Note the transition from active oceanic subduction to collision across the Hormuz strait. (e) Section across the Zagros orogen (location shown in (c)), showing the spectacular deformation of the ZFTB, the presence of high-pressure, low-temperature rocks in the suture zone, and the main domains found in the upper Eurasian plate (i.e. SSZ, Nain–Baft, UDMA, Central Iran; after Agard et al. 2006). Abbreviations: HZF – High Zagros Fault; MFF – Main Front Fault; MZT – Main Zagros Thrust; SSZ – Sanandaj–Sirjan Zone; Tr–J – Triassic to Jurassic cover of SSZ and Central Iran; UDMA – Urumieh–Dokhtar Magmatic Arc. See text (in particular Section 3) for details. (MZT; Fig. 1c; Stöcklin, 1968; Ricou, Braud & foreland deformation, much less is known about the Brunn, 1977; Agard et al. 2005, 2006; Paul et al. hinterland, subduction-related arcs trending parallel to 2006, 2010), is also still discussed (Alavi, 1994; the Zagros, namely the Sanandaj–Sirjan Zone (SSZ) Shafaii Moghadam, Stern & Rahgoshay, 2010). Finally, and the Urumieh–Dokhtar Magmatic Arc (UDMA; compared to our understanding of active tectonics and Fig. 1c). 694 P. AGARD AND OTHERS A number of questions therefore remain unanswered over the whole of Iran. In fact, several other adjacent or unsettled: when did initial collision and shortening domains located in the far-field of Zagros deformation happen, and was it diachronous along strike? How was should be considered whenever studying the Zagros deformation partitioned in space and time? Did one or orogeny (e.g. Jackson, Hains & Holt, 1995): these several slab break-offs occur (i.e. fragmentation and are the Alborz, Kopet Dagh, Central Iran and Sistan sinking of a piece of subducting oceanic slab; von regions (Fig. 1 c, d). We will indeed show that the Blanckenburg & Davies, 1995)? How can the changing history of the Zagros